1. The mRNA expression of H_3 receptor was decreased at hypothalamus and increased at striatum by TCDD administration according to in situ hybridization analysis.2. Identification of new isoform for H3 receptor mRNAThis new isoform had the characteristics described below.(1) This isoform was considered to lack a part of 3^<rd> extracellular loop, 5^<th> transmembrane domain and a part of 3^<rd> intracellular loop.(2) In above region, this isoform was 342 bp shorter than H3 L (wild type isoform).(3) The deletions of this isoform occurred in 3^<rd> exon.(4) The mutation of this isoform was in-flame mutation.3. We found the strain and gender differences of H3 receptor mRNA isoform expression in hypothalamus.Gender difference : H_3 L male < female (in H/W rat, p<0.05)Strain difference : H_3 S, H_<3 (397)> H/W rat < L-E rat (in male, p<0.05)4. The expression of H_3 receptor mRNA was changed by starvation, but not by ovariectomy.5. We found that keton body decrease the of H3 receptor mRNA expression at hypothalamus during the starvation.Our results call for the possibility that the change of H3 receptor mRNA expression involves food-intake and TCDD induces anorexia by decreasing H3 receptor expression.